Various types of circuits have been employed to determine and indicate switch status. Frequently a "debounce" circuit is used, and such an arrangement inspects the switch condition from time to time and then reports on its status. Often the inspection is made by passing a current through the switch. Using a continuous current flow, the probability of error in the state determination is minimal, bu this consumes considerable power. Energy is conserved by shortening the time during which the current is passed through the switch, but this may also increase error in the state determination process, as will be more fully explained hereinafter. It is thus important to attain the optimum trade-off between duration of current flow (that is, power consumption) and proper determination of the switch state.
The inventive system operation is roughly comparable to peeking at a scene or situation, and surmising from a series of peeks what is going on. Suppose someone is watching television, and the family cat is in the corner of the same room. The television watcher occasionally glances at the cat to determine if the cat is sleeping, eating, or playing. The watcher can vary the frequency and/or duration of the glance, and comes to an initial determination of what the cat is doing based on the results noted during the glances. After the first conclusion is made, a further refinement can be added, such as requiring a finite number of successive indications of the same state to conclude what the cat is actually doing. This two step procedure, an initial sensing and a subsequent confirmation, is helpful in gaining a perspective on the invention described below.
Many different types of circuits have been used to examine the state of a given contact set, and report or otherwise signal to associated equipment what is the status of the particular contact. One such example is found in U.S. Pat. No. 4,658,249, entitled "Data Communication System With Key Data Bit Denoting Significance of Other Data Bits", which issued Apr. 14, 1987 to William R. Vogt, the inventor of the subject matter described in this application. This earlier patent depicts in FIG. 7 a "monitored contact" 66, the state of which is examined and reported by a switch state determination arrangement 67. It is to the improvement of such arrangement that the present invention is principally directed. Hence U.S. Pat. No. 4,658,249, and the other patents described in column 1 of that patent, are incorporated herein by reference. In general it is desired to improve upon the arrangement of the '249 patent by providing an improved debounce circuit which not only provides the option of modifying operation of the determination circuit, but in addition effects such modification in accordance with a signal received from an external source.